1. Field of the Invention
The present invention relates to structural panels and more particularly concerns such panels and methods for their manufacture that employ filler materials, having certain desirable properties, together with reinforcing structures to provide a unitary panel structure.
2. Description of Prior Art
Lightweight plastic materials, including the many different types of foamed synthetic resins and expanded plastic foams, such as urethanes, polystyrenes, and the like, have a number of properties that are highly desired in building materials for various types of structures such as walls, roofs, and the like. These properties include light weight, exceedingly low thermal conductivity, resistance to abrasion, impermeability to moisture, and acoustic insulation. However, such materials generally are deficient in structural strength and, therefore, must be combined in some manner with other materials having satisfactory structural properties.
Various configurations employing combinations of lightweight cellular plastic foams or expanded plastic bodies and rigid load-bearing structural elements have been suggested in the past for providing structural building panels that can effectively utilize the desirable properties of the cellular materials. Typical of such prior art arrangements are the U.S. patents to Weisman U.S. Pat. Nos. 3,305,991, 3,555,131 and 3,879,908. In U.S. Pat. Nos. 3,305,991 and 3,555,131, there is described a structural panel and a technique for manufacture thereof in which a three-dimensional reinforcing framework is first built up and then a lightweight plastic core is formed in situ within the framework, and positioned so that the outer surfaces of the resulting foamed in place core are hopefully located inwardly of the outer boundaries of the three-dimensional framework. It is desired to have the framework extend outwardly of the core so that the hybrid panel, the combination of cellular material and rigid framework, may be coated with plaster, gunnite, stucco or the like, in which the projecting portions of the structural framework may be embedded. However, because the cellular material is foamed in place, a form must be provided to define the bottom surface of the foam core. Since this form must be positioned above one outer side of the prefabricated structural framework, difficulties are encountered in maintaining the form in a planar configuration and in positioning the form at a precise distance from the outer surface of the framework.
Further, since no form is generally employed to define the upper surface of the foamed in place core (the panel core is foamed in place within the structural framework, with the panel in a horizontal position according to the teaching of the patents), the foam core will exhibit an outer surface that is far from the desired planar condition. It will exhibit a surface of such irregularity and imprecise location that the foam core may actually contact the outer elements of the structural framework in many places, thereby preventing a subsequent coating from completely encompassing such outer portions of the framework.
The later patent to Weisman U.S. Pat. No. 3,879,908 avoids some problems of the use of foamed in place material and instead, after building a completed three-dimensional structural framework that covers the entire panel on all six sides thereof, inserts a plurality of insulative elements through passages that are disposed wholly within the structural framework. These insulative elements must be dimensioned so as to freely and easily pass between adjacent elements of the structural framework and, when positioned, will not have adjacent surfaces in contact with one another. Impermeability to moisture is thereby greatly degraded. Thereafter a bonding agent is employed in the arrangement of the Weisman U.S. Pat. No. 3,879,908 to secure the insulative elements in position within the lattice. This bonding agent is a layer of material that is foamed in place, but in a somewhat thinner layer than the remainder of the core, and is employed to bond the insulative elements to the metal elements of the structural framework. This is a time-consuming and expensive procedure, and still provides a panel surface formed by a foamed in place bonding agent. Such surface may be irregular and improperly positioned with respect to the outer surface of the structural framework. Further, since the insulative elements must be inherently spaced from one another in order to allow them to be inserted into the passages in the structural framework, they can form no vapor or moisture barrier. Their heat and sound insulating properties also are significantly degraded by virtue of the space between adjacent insulating elements, even though the ends of such spaces are covered by the foamed in place bonding agent.
Because of its extremely low thermal conductivity, foamed urethane is potentially a particularly desirable core material. However, it is subject to deterioration in the form of crumbling when exposed to air and light for extended periods. In the manufacture and use of structural panels such as those described above, such prolonged exposure frequently occurs during lengthy storage periods between fabrication and installation of the panels. The deterioration of the urethane core material is substantially accelerated by the common practice of storing the completed panels outdoors where they are directly exposed to the elements while they await shipment to the job site and installation of their protective layers of plaster, stucco or the like. For this reason it has, in the past, proven more practical to form the panel cores from materials (such as polystyrene) which are much less subject to such deterioration--despite their higher thermal conductivities.
Yet another problem exists in the panel construction techniques disclosed in the Weisman patents. Both the foamed in place method and the element insertion and bonding method preclude, as a practical matter, the addition of foil facing to the opposite side of the expanded plastic core--either before or after the core is formed or placed within its supporting lattice structure. Such facing is a common and desirable addition to sheets of expanded plastic insulating material used without supporting lattice structures in that it provides reflective barriers to radiant heat transfer through the insulation and adds two more layers of moisture impermeable material to the sheet. In the case of urethane insulation, the foil facing (since it leaves only the edges of the sheet exposed) additionally affords significant protection from deterioration during extended exposure to light and air.
In the foamed in place method taught by Weisman, it is readily apparent that the addition of foil facing to the panel core is neither economically or practically feasible--both because of the interfering lattice structure and the resulting irregular surface of the expanded plastic itself. Moreover, in the element insertion and bonding method, the application of foil facing to the elements (prior to or subsequent their insertion in the lattice structure) would greatly hinder, if not preclude, the addition of the necessary bonding agent to the elements.
Accordingly, it is an object of the present invention to provide a composite structural panel that avoids or minimizes above-mentioned and other problems.